If you split it up by platform, then SMS usage is larger than an individual IM app. So unless everyone in the world somehow agrees on one IM app that everyone will use, SMS will still always be relevant and useful.
This is not true at all, on both fronts. SMS usage totals about 1.8 trillion messages per year sent globally, 70% of which is generated by two regions and really it's almost entirely two countries. Whatsapp has 20 trillion messages per year and over a billion users using it on a daily basis. In most countries, SMS is used almost exclusively by businesses. Daily users of SMS add up to about 60% of the number of users as WhatsApp, with less than 10% of the daily traffic in quantity of messages. WhatsApp is also used for sharing video, photos, calls, etc. and that traffic alone is the same volume as SMS traffic and is on top of the 20 trillion messages per year. WhatsApp is on almost twice as many phones as iMessage is, as is WeChat and FB Messenger, etc.
If we were going to use volume or availability to users, then WhatsApp would be what we have to consider to be the global "standard" - which explains why Allo, as well as all the other services that you mentioned, all want to be so much like WhatsApp.
Seriously, the only reason that people in the US think that SMS is popular, is because SMS is typically bundled in for free with most carrier plans in the US - making it a cheap or free service, while the US also has very expensive data plans (until very recently) and therefore SMS was the more economical resource for the majority of Americans. SMS usage is declining in the US and has been since 2011, but at a much slower rate than in the rest of the world, where SMS usage is plummeting year over year and has been since 2010, when smartphones became popular. The reason for that is that, in most countries, data is cheaper than SMS - which is typically a pay per message resource. Using a tiny amount of data, which includes a MUCH better user experience, is almost always cheaper than using SMS and that's including countries where incoming SMS are free, such as most of the EU.
Almost all smartphone plans in Italy have a limit on the number of SMS you can send in a day (and a monthly cap as well), which varies from 10 to 1000, depending on the price of your plan and increasing the SMS allotment almost always includes accepting a lower cap on your data. After reaching the limit, the cost of each SMS message can be anywhere from €0.04 to €0.20. But an important thing to keep in mind with that pricing, before anyone says, "sms should be free" - that phone plan is typically around €16 per month per person, compared with my Verizon plan in the US, where I'm paying the equivalent of €62 per person. That's almost 4 times as expensive.
Now that the big 3 carriers and Sprint have all started promoting the hell out of "unlimited" data plans, SMS usage will start to plummet just as much here as it did for the rest of the planet 8 years ago.
In the US, FB messenger is already more widely used both in terms of # of people and total volume of messages, than SMS is and for some reason it is much more popular in the US than WhatsApp, though both are owned by the same company. So as a global standard, we already have WhatsApp - though many apps want to challenge it. In China, we have WeChat, in the US we have FB Messenger - SMS is actually the king of the hill in exactly nowhere and isn't even in the top 10 services being used.
Additionally, there are apps like Franz and Clatter, etc. which combine all of these different IM services into one interface for you (desktop applications). A similar app can be found on Android called Disa
https://play.google.com/store/apps/details?id=com.disa&hl=en, though WhatsApp occassionally breaks their plugin. Either way, you get the idea and having unified messengers makes having contacts that are all on separate messengers suck less.